The present invention relates to an electrostatic charging system for atomizers and coating applicators; more particularly, the invention relates to an ionizing system adapted for use in connection with an electrostatic paint applicator. The electrostatic paint applicator may be either a hand-held spray gun or may be an automatic spray gun which is operable by remote control connections, or a paint powder applicator. The invention is primarily useful for applying non-conductive liquids and powders, although the principles of the invention also find use in connection with spraying conductive liquids.
In the field of electrostatic spraying, it is desirable to create an electrostatic field in the vicinity between the spray gun and the target or article to be sprayed. The sprayed particles are propagated through this field, and the respective particles pick up voltage charges as they pass through the field. The charged particles are thereby attracted to the article to be sprayed, which is typically maintained at a ground or zero voltage potential so as to create an attractive force between the grounded article and the charged particles. By this process, it is possible to direct a much higher percentage of sprayed particles to the actual article to be sprayed, and thereby the efficiency of spraying is vastly improved over conventional methods.
In a typical electrostatic spraying system, an ionizing electrode is placed in the vicinity of the spray gun spray orifice, the article to be painted is held at ground potential, and an electrostatic field is developed between the ionizing electrode and the article. The distance between the two electrodes may be on the order of about one foot; therefore, the voltage applied to the spray gun electrode must necessarily be quite high in order to develop an electrostatic field of sufficient intensity to create a large number of ion/particle interactions so as to develop a sufficient attractive force between the paint particles and the target. It is not unusual to apply electrostatic voltages on the order of 60,000-100,000 volts (60-100 kv) to the spray gun electrode in order to achieve a proper degree of efficiency in the spraying operation. An ionizing current on the order of 50 microamps typically flows between the grounded article and the spray gun electrode.
Electrostatic systems of the foregoing type are frequently referred to as corona charging systems, because the field intensity creates a corona current from the electrode which ionizes the air in the vicinity, and the atomized paint particles which pass through the region of ionized air pick up the ionized charges and become more readily attracted to a grounded or neutral article to be coated. The efficiency of this process can be determined by the number of ions n which are applied to a typical particle as it passes between the spray gun and the target, according to the relationship EQU n=k*E*t*I;
where:
n=number of ion charges per droplet; PA1 k=constant PA1 E=electrical field strength in the charging zone; PA1 t=time the droplet is in the charging zone; PA1 I=ion concentration in the charging zone.
The electrical field strength in the charging zone must be sufficiently intensive as to ionize the air in the vicinity of the electrode (in the charging zone) in order to create the corona current described above.
Electrostatic voltage charging systems can be utilized in connection with spray guns whether the primary atomizing forces are created by pressurized air, hydraulic forces, or centrifugal forces. In each case, it is preferable that the ionizing electrode be placed at or proximate to the point where atomization occurs so as to cause the greatest number of atomized particles to pass through the ionizing field. Electrostatic ionizing systems can also be used with conductive or nonconductive paint; but in the case of conductive paint, the placement of the electrostatic ionizing electrode may have to be more carefully positioned so as to avoid developing a conductive path through the liquid paint column prior to the point of atomization. In the prior art, the electrostatic electrode configuration most often used for satisfactory performance is a needle configuration, which permits a high intensity field to develop at the needle tip, wherein the needle is positioned at or proximate to the zone of atomization. In the prior art, these needles are typically made from hardened steel material, frequently stainless steel, typically having a diameter of about 0.5 millimeters (mm) and projecting forwardly from the nozzle a distance of about 2-6 mm. These needles are typically formed from wire material which is cut to length, and no attempt is made to provide a sharpened point on the needle. In some cases the needle end is rounded. The voltage as applied to such needles is usually in the range of 40-100 kv, which develops a relatively high intensity electrostatic field in the vicinity of the needle, wherein the electrostatic field lines are formed between the needle and usually a grounded article to be painted. The field gradient in volts per centimeter (v/cm) is determined by dividing the voltage applied to the needle by the distance in centimeters to the second electrode, usually the article, where the field is developed.
It would be a distinct advantage in the field of electrostatic spraying to provide a construction having a very high electrostatic field intensity with a considerably lower applied voltage than as used in the prior art. For example, reducing the applied voltage from 60 kv to 15 kv greatly simplifies the technical design of the voltage-producing circuitry, reduces the complexity of shielding the electrostatic field from adverse outside influences, and increases the overall safety in operating the system. The factors that can influence the design of an appropriate electrostatic system include the distance between the respective electrodes, the geometry of the electrodes, the position of the electrodes relative to the atomized spray, and the type of material sprayed by the system.